U.S. patent number 7,142,619 [Application Number 09/843,161] was granted by the patent office on 2006-11-28 for long subscriber loops using automatic gain control mid-span extender unit.
This patent grant is currently assigned to Symmetricom, Inc.. Invention is credited to Gary Bogardus, Kamila Kraba, Kishan Shenoi, Jeremy Sommer, Sandro Squadrito.
United States Patent |
7,142,619 |
Sommer , et al. |
November 28, 2006 |
Long subscriber loops using automatic gain control mid-span
extender unit
Abstract
Systems and methods are described for long subscriber loops
using automatic gain control. A method includes extending a digital
subscriber loop including: producing an output signal in a first
direction from a variable gain amplifier at a mid-span extender
unit responsive to an input signal in the first direction from the
digital subscriber loop; monitoring a signal strength of said
output signal in the first direction at the mid-span extender unit;
generating a gain control signal responsive to the signal strength
at the mid-span extender unit; controlling a gain of the variable
gain amplifier at the mid-span extender unit responsive to the gain
control signal; and controlling a second gain of a second variable
gain amplifier at said mid-span extender unit responsive to said
gain control signal to produce an output signal in a second
direction from said second variable gain amplifier at said mid-span
extender unit responsive to a second input signal in said second
direction from said digital subscriber loop.
Inventors: |
Sommer; Jeremy (Mountain View,
CA), Shenoi; Kishan (Saratoga, CA), Kraba; Kamila
(Santa Clara, CA), Squadrito; Sandro (San Jose, CA),
Bogardus; Gary (San Carlos, CA) |
Assignee: |
Symmetricom, Inc. (San Jose,
CA)
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Family
ID: |
26895303 |
Appl.
No.: |
09/843,161 |
Filed: |
April 25, 2001 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20020105964 A1 |
Aug 8, 2002 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60199930 |
Apr 26, 2000 |
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Current U.S.
Class: |
375/345; 375/213;
375/224; 375/220; 375/211 |
Current CPC
Class: |
H04B
3/06 (20130101); H04L 25/24 (20130101) |
Current International
Class: |
H04L
27/08 (20060101) |
Field of
Search: |
;375/345,247,220,224,211,213 ;320/18.1,13.3,226,315,463 ;379/417
;455/127.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 338 804 |
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Oct 1989 |
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EP |
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2 337 380 |
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Nov 1999 |
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GB |
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0065094 |
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Aug 1999 |
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KR |
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4, 2000. cited by other.
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Primary Examiner: Fan; Chieh M.
Assistant Examiner: Wang; Ted M.
Attorney, Agent or Firm: Bruckner PC; John
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to, and claims a benefit of priority
under 35 U.S.C. 119(e) and/or 35 U.S.C. 120 of U.S. Ser. No.
60/199,930, filed Apr. 26, 2000, now pending, the entire contents
of which are hereby incorporated by reference for all purposes.
Claims
What is claimed is:
1. A method, comprising extending a digital subscriber loop
including: producing an output signal in a first direction from a
first variable gain amplifier at a mid-span extender unit
responsive to an input signal in the first direction from said
digital subscriber loop; monitoring a signal strength of said
output signal in the first direction at said mid-span extender
unit; generating a gain control signal responsive to said signal
strength at said mid-span extender unit; controlling a first gain
of said first variable gain amplifier at said mid-span extender
unit responsive to said gain control signal; controlling a second
gain of a second variable gain amplifier at said mid-span extender
unit responsive to said gain control signal to produce an output
signal in a second direction from said second variable gain
amplifier at said mid-span extender unit responsive to a second
input signal in said second direction from said digital subscriber
loop; and detecting whether a downstream signal is present on said
digital subscriber loop, wherein controlling said gain of said
first variable gain amplifier includes determining when to change
said gain of said first variable gain amplifier based on at least
one elapsed time interval selected from the group consisting of
Tnormal, Tshutdown, Tsleep, and Tdead, where Tnormal is a duration
that persists while i) said downstream signal is present, and said
gain of said first variable gain amplifier is between a lower
threshold and an upper threshold, or ii) said downstream signal is
present, an upstream power level is below a lower threshold, and
said gain of said first variable gain amplifier is at an upper
limit, or iii) said downstream signal is present, said upstream
power level is above an upper threshold, and said gain of said
first variable gain amplifier is at a lower limit, where Tshutdown
is a maximum duration of link termination, where Tsleep is a
subsequent duration, and where Tdead is a duration that persists
with a same gain setting while said downstream signal is not
present and a control signal is below a low threshold.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to the field of communications.
More particularly, the invention relates to digital subscriber loop
(DSL) communications. Specifically, a preferred implementation of
the invention relates to extending the range of an asymmetric
digital subscriber loop (ADSL). The invention thus relates to ADSL
of the type that can be termed extended.
2. Discussion of the Related Art
Conventional telephony, often called plain old telephone service
(POTS), is provided to customers over copper cable. This copper
cable can be termed a subscriber loop or a subscriber line. Modern
loop plant designs specify the use of 26-gauge cable for short to
medium loop lengths with 24-gauge cable used to extend the range.
Legacy loop plant includes cable of 22-gauge as well as
19-gauge.
At the customer premises, a telephone set is typically connected to
the cable. The other end of the cable is connected to a line
circuit module in the service provider's central office (CO).
Switches terminating customer loops at the central office are
regarded as Class-5 switches and provide a dial-tone. The customer
premise equipment (CPE) can include a personal computer (PC)
modem.
Older central office switches were analog in nature and were unable
to provide a broad range of services. Modern central office
switches are digital. Digital switches include codecs in the line
circuit to do the bilateral analog-digital (A/D) conversion; the
transmission over the loop is analog and the signals occupy a
frequency band of up to (approximately) 4 kHz. Conventional
telephony codecs convert at an 8 kHz sampling rate and quantize to
8 bits per sample corresponding to a net bit rate of 64 kbps (or
"DS0").
With the advent of digital terminal equipment, such as personal
computers, modems were developed to carry digital bit streams in an
analog format over the cable pair. Because of the 4 kHz constraint
imposed by the A/D converter in the line circuit, the data rate of
such transmission is limited and is typically 9.6 kbps. More
elaborate schemes have been proposed which permit higher bit rates
(e.g. V.34 which can do in excess of 28.8 kbps). More recently,
there are schemes that "spoof" the D/A converter in the
line-circuit and operate at bit rates as high as 56 kbps in the
downstream direction (from CO to CPE). With increasing deployment
of, and consequently demand for, digital services it is clear that
this bit rate is insufficient.
An early proposal to increase the information carrying capacity of
the subscriber loop was ISDN ("Integrated Services Digital
Network"), specifically the BRI ("Basic Rate Interface") which
specified a "2B+D" approach where 2 bearer channels and one data
channel (hence 2B+D) were transported between the CO and the CPE.
Each B channel corresponded to 64 kbps and the D channel carried 16
kbps. With 16 kbps overhead, the loop would have to transport 160
kbps in a full duplex fashion. This was the first notion of a
Digital Subscriber Loop ("DSL") (or Digital Subscriber Line).
However, this approach presumed that POTS and 2B+D would not
coexist (simultaneously). The voice codec would be in the CPE
equipment and the "network" would be "all-digital". Most equipment
was designed with a "fall-back" whereby the POTS line-circuit would
be in a "stand-by" mode and in the event of a problem such as a
power failure in the CPE, the handset would be connected to the
loop and the conventional line-circuit would take over. There are
several ISDN DSLs operational today..sup.(1-2)
Asymmetric digital subscriber loop (ADSL) was proposed to provide a
much higher data rate to the customer in a manner that coexisted
with POTS. Recognizing that the spectral occupancy of POTS is
limited to low frequencies, the higher frequencies could be used to
carry data (the so-called Data over Voice approach). Nominally,
ADSL proposed that 10 kHz and below would be allocated to POTS and
the frequencies above 10 kHz for data. Whereas the nominal ADSL
band is above 10 kHz, the latest version of the standard specifies
that the "useable" frequency range is above 20 kHz. This wide band
between 4 kHz and the low edge of the ADSL band simplifies the
design of the filters used to segregate the bands.
Furthermore, it was recognized that the downstream data rate
requirement is usually much greater than the upstream data rate
requirement. Several flavors ("Classes") of ADSL have been
standardized, involving different data rates in the two directions.
The simplest is Class-4 which provides (North American Standard)
1.536 Mbps in the downstream direction and 160 kbps in the upstream
direction. The most complicated, Class-1, provides about 7 Mbps
downstream and 700 kbps upstream..sup.(3-4)
A stumbling block in specifying, or guaranteeing, a definite bit
rate to a customer is the nature of the loop plant. Customers can
be at varied geographical distances from the central office and
thus the length of the subscriber loop is variable, ranging from
short (hundreds of feet) to long (thousands of feet) to very long
(tens of thousands of feet). The essentially lowpass frequency
response of subscriber cable limits the usable bandwidth and hence
the bit rate.
Moreover, loops longer than (approximately) 18 thousand feet have a
lowpass characteristic that even affects the voiceband. Such loops
are specially treated by the addition of load coils and are called
"loaded loops". The principle is to splice in series-inductors
which have the impact of "boosting" the frequency response at
(approximately) 4 kHz with the secondary effect of increasing the
attenuation beyond 4 kHz very substantially. In these loaded loops,
the spectral region above 10 kHz is unusable for reliable
transmission. Consequently, the categorical statement can be made
that DSL (including ADSL, "2B+D", and other flavors of DSL) cannot
be provided over long loops and definitely cannot be provided over
loaded loops.
Heretofore, there has not been a completely satisfactory approach
to providing DSL over long loops. Further, there has not been a
satisfactory approach to providing DSL over loaded loops. What is
needed is a solution that addresses one, or both, of these
requirements. The invention is directed to meeting these
requirements, among others.
SUMMARY OF THE INVENTION
There is a need for the following embodiments. Of course, the
invention is not limited to these embodiments.
One embodiment of the invention is based on a method, comprising:
extending a digital subscriber loop including: producing an output
signal from said first variable gain amplifier responsive to an
input signal from said digital subscriber loop; monitoring a signal
strength of said output signal; generating a gain control signal
responsive to said signal strength; and controlling a gain of said
variable gain amplifier responsive to said gain control signal.
Another embodiment of the invention is based on an apparatus,
comprising a digital subscriber loop extender circuit including: a
variable gain amplifier having a gain and providing an output
signal in response to an input signal from a signal generator over
a transmission medium, said output signal having a signal strength
as a function of said gain; and a controller coupled to said
variable gain amplifier, said controller generating a gain control
signal that is feed back to said variable gain amplifier to
automatically control said gain.
These, and other, embodiments of the invention will be better
appreciated and understood when considered in conjunction with the
following description and the accompanying drawings. It should be
understood, however, that the following description, while
indicating various embodiments of the invention and numerous
specific details thereof, is given by way of illustration and not
of limitation. Many substitutions, modifications, additions and/or
rearrangements may be made within the scope of the invention
without departing from the spirit thereof, and the invention
includes all such substitutions, modifications, additions and/or
rearrangements.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawings accompanying and forming part of this specification
are included to depict certain aspects of the invention. A clearer
conception of the invention, and of the components and operation of
systems provided with the invention, will become more readily
apparent by referring to the exemplary, and therefore nonlimiting,
embodiments illustrated in the drawings, wherein like reference
numerals (if they occur in more than one view) designate the same
elements. The invention may be better understood by reference to
one or more of these drawings in combination with the description
presented herein. It should be noted that the features illustrated
in the drawings are not necessarily drawn to scale.
FIG. 1 illustrates a block schematic view of the more important
components of an ADSL repeater equipped subscriber loop,
representing an embodiment of the invention.
FIG. 2 illustrates a block schematic view of the more important
elements of a DMT signal processing flow (echo canceling mode),
representing an embodiment of the invention.
FIG. 3 illustrates a block schematic view of a frequency-division
duplexing mode for DMT-based ADSL (central office end shown),
representing an embodiment of the invention.
FIG. 4 illustrates a block schematic view of an exemplary
asymmetric digital subscriber loop repeater, representing an
embodiment of the invention.
FIG. 5 illustrates a block schematic view of an outline of an
extender circuit, representing an embodiment of the invention.
FIG. 6 illustrates a block schematic view of an equivalent circuit
for depicting automatic gain control operation, representing an
embodiment of the invention.
FIG. 7 illustrates a block schematic view of a bi-directional ADSL
repeater with upstream AGC, representing an embodiment of the
invention.
FIG. 8 illustrates a flow diagram of a process that can be
implemented by a computer program, representing an embodiment of
the invention.
FIG. 9 illustrates a flow diagram of a wait or force auxiliary
operation that can be implemented by a computer program,
representing an embodiment of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
The invention and the various features and advantageous details
thereof are explained more fully with reference to the nonlimiting
embodiments that are illustrated in the accompanying drawings and
detailed in the following description. Descriptions of well known
components and processing techniques are omitted so as not to
unnecessarily obscure the invention in detail. It should be
understood, however, that the detailed description and the specific
examples, while indicating preferred embodiments of the invention,
are given by way of illustration only and not by way of limitation.
Various substitutions, modifications, additions and/or
rearrangements within the spirit and/or scope of the underlying
inventive concept will become apparent to those skilled in the art
from this detailed description.
Within this application several publications are referenced by
Arabic numerals within parentheses or brackets. Full citations for
these, and other, publications may be found at the end of the
specification immediately preceding the claims after the section
heading References. The disclosures of all these publications in
their entireties are hereby expressly incorporated by reference
herein for the purpose of indicating the background of the
invention and illustrating the state of the art.
The below-referenced U.S. Patent Applications disclose embodiments
that were satisfactory for the purposes for which they are
intended. The entire contents of U.S. patent application Ser. No.
09/476,770, filed Jan. 3, 2000; U.S. patent application Ser. No.
09/821,841, filed Mar. 28, 2001, (now U.S. Pat. No. 6,507,606);
U.S. patent application Ser. No. 09/836,889, filed Apr. 16, 2001,
(now abandoned); and U.S. patent application Ser. No. 09/838,575,
filed Apr. 18, 2001, (now U.S. Pat. No. 6,842,426) are hereby
expressly incorporated by reference herein for all purposes.
The context of the invention includes digital subscriber loops. One
species of digital subscriber loops is an asymmetrical digital
subscriber loop. A preferred embodiment of the invention using ADSL
repeaters (in place of load coils) enables a form of ADSL that uses
the technique of frequency-division-duplexing to be provided to
customers over very long loops.
The agreed upon standard for ADSL is the DMT (Discrete Multi-Tone)
method. A premise underlying DMT is that the channel, namely the
subscriber loop, does not have a "flat" frequency response. The
attenuation at 1 Mhz ("high" frequency) can be as much as 60 dB
greater than at 10 kHz ("low" frequency). Furthermore this
attenuation varies with the length of the cable. By using Digital
Signal Processing ("DSP") techniques, specifically the theory of
the Discrete Fourier Transform ("DFT") and Fast Fourier Transform
("FFT") for efficient implementation, the DMT method splits the
available frequency band into smaller sub-channels of
(approximately) 4 kHz. Each sub-channel is then loaded with a data
rate that it can reliably support to give the desired aggregate
data rate. Thus lower (center-)frequency sub-channels will normally
carry a greater data rate than the sub-channels at higher
(center-)frequencies.
The underlying principle of the DSL repeater is the need to combat
the loss in the actual cable (subscriber loop). This is achieved by
introducing gain. Since amplifiers are for the most part
uni-directional devices, one approach is to perform a 2w-to-4w
conversion and put amplifiers in each direction. This is most
easily achieved when the directions of transmission are in disjoint
spectral bands. That is, if the directions of transmission are
separated in frequency (i.e. frequency-division duplexing), then
simple filter arrangements can provide the separation.
Most loop plant provides for access to the cable, which may be
buried underground, approximately every 6000 feet. This was the
practice to allow for the provision of load coils. Thus the natural
separation between repeaters is (approximately) 6000 feet. The
repeater may be placed in parallel with a load coil if the DSL
needs to coexist with POTS.
Referring to FIG. 1, a general architecture for providing an
asymmetric digital subscriber loop (ADSL) is depicted. A subscriber
loop is the actual two-wire copper pair that originates at the
Central Office and terminates at the subscriber's premise. For
providing ADSL over long loops, an ADSL repeater, 100, may be
included. At the customer premise the handset (POTS) is "bridged"
onto the subscriber loop at point labeled S1. In some forms of ADSL
this bridging can be achieved using passive filters (called a
"splitter") to demarcate the frequency bands where voice and data
reside. Similarly, a splitter may be employed at the central office
(CO) at point S2. Central office equipment that interfaces to ADSL
provisioned lines is often embodied as a multiplexer called a
"DSLAM" (Digital Subscriber Line Access Multiplexer). The data
component is aggregated into an optical or high-bit-rate signal for
transport to the appropriate terminal equipment. The capacity of
ADSL allows for additional voice circuits (shown as VF in FIG. 1)
to be carried in digital format as part of the ADSL data stream.
This content is usually (though not always) destined for a Class-5
switch.
The term approximately, as used herein, is defined as at least
close to a given value (e.g., preferably within 10% of, more
preferably within 1% of, and most preferably within 0.1% of). The
term coupled, as used herein, is defined as connected, although not
necessarily directly, and not necessarily mechanically. The term
substantially, as used herein, is defined as at least approaching a
given state (e.g., preferably within 10% of, more preferably within
1% of, and most preferably within 0.1% of).
Given that a large installed loop plant exists, the invention can
include retrofit installation. Part of the retrofit installation
procedure involves removal of all load coils, and bridge-taps that
may be present on the (existing) subscriber loop. Based on
telephone company records, the (approximate) distance between the
subscriber premise and the serving Central Office can be estimated
to decide whether DSL can be provided in the first place. If DSL
can indeed be provided, an estimate of the class (and thus the data
carrying capacity) is made. If not, then the telephone company may
choose to provide a lower bit-rate service such as BRI or, in some
cases, not be able to provide any service beyond POTS.
Signals from both directions can coexist on the cable pair and such
transmission is referred to as "2-wire". This form is perfectly
adequate for analog signals (speech). In digital transmission
systems the two directions are separated (logically, if not
physically) and such transmission is termed "4-wire". Two common
approaches to achieving this action are "echo canceling" and
frequency-division-duplexing ("FDD"). Both approaches can be
supported by the DMT method.
Referring to FIG. 2, a signal processing flow in a DMT-based ADSL
transmission unit ("ATU") that employs echo cancellation is
depicted. The transmit ("modulation" direction) side is considered
first. The data to be transmitted is first processed to include
error correction by a ENC. & DEC. & ERR. & ETC. unit.
It is then formatted into multiple "parallel" channels via a PARRL
processing unit. It is then placed in the appropriate frequency
slot via a FFT processing unit. The notion of "cyclic extension" is
unique to DMT and involves increasing the sampling rate by
insertion of additional samples via a CYC. EXT. processing unit.
This composite signal is converted to analog via a D/A converter
and coupled to the line via a 2w-to-4w converter. An ADSL repeater
200 is coupled to the 2w-to-4w converter.
Ideally the entire signal from the D/A converter is transmitted to
the distant end via the 2w-to-4w converter. However, in practice
some amount "leaks" from the 2w-to-4w converter toward a A/D
converter. This leakage can be termed the "echo."
The receive side ("demodulation" direction) is now considered. The
signal from the distant end arrives at the 2w-to-4w converter via
the repeater 200 and is directed to the A/D converter for
conversion to digital format. Subsequent processing includes line
equalization via the LINE EQU. unit, fast Fourier transformation
via the FFT unit and then channel equalization and data detection
via the CHAN. EQU. & DET. unit. Processing is then handed to
the unit that does the error detection and/or correction and
reorganizing into the appropriate format. To remove the echo (the
component of the transmit signal that leaks across the 2w-to-4w
converter) an echo cancellation filter is employed. This is a
digital filter that mimics the echo path and thus the output of the
filter labeled "Echo Canc" is a "replica" of the echo and by
subtraction of this signal from the received signal at a summation
unit, the net echo can be substantially reduced. Thus 4w operation
is achieved even though the medium is merely 2w. The spectral
content of signals in the two directions can have significant
overlap but are sufficiently separated by the echo cancellation
technique.
Referring to FIG. 3, a frequency-division duplexing (FDD) mode of
DMT for ADSL is depicted. The "back-end" of the FDD version of
DMT-based ADSL is substantially the same as the echo-canceling
version illustrated in FIG. 2.
Referring again to FIG. 3, the frequency range used for Upstream
versus Downstream is vendor specific. Standards-compliant ADSL uses
a total bandwidth of roughly 20 kHz to 1.1 MHz. In a preferred
embodiment, the upstream occupies between 20 kHz and X.sub.1 kHz
whereas the downstream signal occupies the band between X.sub.2 kHz
and 1.1 MHz. X.sub.2 should be substantially greater than X.sub.1
to allow for frequency roll-off of the filters used to demarcate
the upstream and down-stream bands. One suitable choice is
X.sub.1=110 kHz and X.sub.2=160 kHz. The specific choice of these
band edges can be made a design parameter and different "models" of
the repeater can be fabricated with different choices of band
edges.
Still referring to FIG. 3, a high pass filter HPF unit is coupled
to the D/A units. A 2w-to-4w converter is coupled to the HPF unit.
The 2w-to-4w converter is also coupled to a low pass filter LPF
unit which is in-turn coupled to the A/D unit. An ADSL repeater 300
is coupled to the 2w-to-4w converter.
The underlying principle of the ADSL extender is the need to combat
the loss in the actual cable (subscriber loop). This is achieved by
introducing gain. Since amplifiers are for the most part
unidirectional devices, we need to, in essence, perform a 2w-to-4w
conversion and put amplifiers in each direction. This is most
easily achieved when the directions of transmission are in disjoint
spectral bands. That is, if the directions of transmission are
separated in frequency (i.e. frequency-division duplexing), then
simple filter arrangements can provide the separation.
Most loop plant provide for access to the cable, which may be
buried underground, approximately every 6000 feet. This was the
practice to allow for the provision of load coils. Thus, the
natural separation between repeaters is (approximately) 6000 feet.
The repeater may be placed in parallel with a load coil if the ADSL
needs to coexist with POTS.
The particular description of an ADSL repeater provided in FIG. 4
is suitable for the DMT-based ADSL transmission scheme employing
frequency-division duplexing (FDD). The form discussed assumes that
POTS and ADSL will coexist (simultaneously). Of course, the
invention is not limited to this ADSL FDD example.
Referring to FIG. 4, an outline of the functional blocks in an ADSL
repeater 400 are depicted. For convenience certain functions such
as power and control are not shown in FIG. 4. Power and control
units can be coupled to the ADSL repeater 400. Although not
required, two load coils are shown as part of the repeater 400.
When load coils are deployed in a loop, the loop is split and the
load coils are spliced in as indicated by the series connections of
the inductors (load coils) with the loop. This can be termed in
line with loop.
The load coils provide a very high impedance at high frequencies
and thus for the range of frequencies where ADSL operates the load
coils look essentially like open circuits. The 2w-to-4w arrangement
is not explicitly shown in FIG. 4 but is implied. Since the two
directions are separated in frequency, the 2w-to-4w arrangement can
be quite simple. A bandpass filter BPF isolates the frequency band
from 20 kHz to 110 kHz (approximately) and thus the upstream signal
is amplified by an amplifier AMP-U. In this particular example, the
gain introduced can compensate for the attenuation introduced by
approximately 6000 feet of cable at 27 kHz (or approximately the
middle of the band). The highpass filters HPF separates out the
band above 160 kHz (approximately) and thus the downstream signal
is amplified by an amplifier AMP-D. Again, in this particular
example, the gain introduced compensates for the attenuation of
approximately 6000 feet of cable at 600 kHz (again, roughly the
middle of the band).
Since the frequency response of the cable is not "flat" the
amplifiers can be designed such that, in conjunction with the
filters, they provide a rough amplitude equalization of the cable
response over the appropriate frequency band, for example,
approximately 20 kHz to 110 kHz upstream and approximately 160 kHz
to 1 MHz downstream. The choice of frequency bands is, preferably,
20 kHz to 110 kHz for the upstream direction and 160 kHz to 1.1 MHz
for the downstream direction.
If POTS need not be supported, then the load coils are superfluous
and can be left "open". Further, if the need for load coils is
obviated, the separation of the units becomes a design parameter,
independent of load coil placement. A suitable separation of
Extenders in this situation is between 7 and 12 kft, and the unit
can then be referred to as a "Mid-Span Extender". Clearly, the
gains required for the mid-span extender are commensurate with the
expected separation.
An ADSL Repeater is well suited for providing ADSL services over
long loops which may have been precluded based on loop length and
presence of load coils. As described it is a simple mechanism for
amplifying the upstream and downstream signals, compensating for
the loss in the subscriber loop cable. Separating repeaters by
approximately 6000 feet is appropriate since this the nominal
distance between points on the cable where load coils were
introduced in the past. Cross-over networks based on highpass and
bandpass filters can define the upstream and downstream bandwidths
used by the DMT-based ADSL units at the CO and CPE operating in a
frequency-division duplex mode.
Installing equipment in the cable plant introduces two important
considerations. One is the need to provide power. The second is to
provide the means to verify operation and isolate problems.
Subscriber loop cable usually comes in bundles of 25 pairs. That is
each bundle can provide service to 25 telephone lines. One
embodiment of the invention can use the 25 pairs to provide just 20
ADSL connections. This leaves 4 pairs to carry power for the
repeaters, and 1 pair to carry control information.
Each 25-pair "repeater housing" can include one controller
(microprocessor) and modems that convert the digital control
information to (and from) analog for transport over the control
pair. These controllers can operate in a "daisy chain" which allows
the central office end to query for status, or control the
operation of, any repeater housing in the path. For long loops,
those exceeding 18 thousand feet, there may be as many as 4 or 5
(or more) repeater housings connected in series (approximately 6000
feet apart). The control information will include commands for
maintenance and provisioning information.
The provisioning information relates to the mode of operation of
each of the 20 pairs of cable that carry ADSL. One mode is
"normal", where the repeater is operating and the load coils are in
the circuit. Another mode is "no-ADSL-repeater" wherein the
repeaters are not part of the circuit. This latter mode has two
"sub-modes". The load-coils may be in the circuit or be removed.
The last sub-mode is appropriate if the loop is actually short and
we do not need the repeaters and the load coils need to be removed.
Of course, other modes of operation can be conceived of.
For test and maintenance purposes, the central office end needs to
be capable of forcing any one chosen repeater (on the subscriber
loop under test) to enter a loop-back state. That is, a test signal
sent from the central office is "looped back" at the chosen
repeater and the condition of the loop up to that chosen repeater
can be validated. Other test and maintenance features must be
provided to support the operating procedures of the phone
company.
For providing loop-back through the repeater, the following
approach can be used. It can be appreciated that the upstream and
downstream signal bands are disparate and non- overlapping. Thus,
the notion of loop-back is not simple. One approach can use a
two-tone test signal that is within the downstream spectral band.
For example, the tone frequencies could be 200 kHz and 250 kHz.
When commanded to go into loop-back, the designated repeater
introduces a nonlinear element into the circuit. The nonlinear
element will create different combinations of the sums and
difference frequencies. In particular, the nonlinear element can
generate the difference frequency, 50 kHz in the example cited.
This signal is within the frequency band of the upstream direction
and thus can be looped back. The central office end can monitor the
upstream path for this (difference) frequency and thus validate the
connectivity up to the repeater in loop-back state.
The form of extender where load coils are not being replaced is the
mid-span extender. Placement of a mid-span extender is not
constrained by the placement of load coils but, as a matter of
practice, the phone company usually has a manhole or equivalent
construction where load coils are (normally) situated and these
locations would be logical places for deployment of a mid-span
extender as well. When a mid-span extender is employed, the load
coil removal would follow normal telephone company practice.
The basic circuit outline 500 of the extender unit is shown in FIG.
5. The extender unit includes a first 2w-4w and a second 2w-4w. For
the case of a "load coil replacement", the 88 mH inductors 510
would be present and the gains adjusted for compensating for
(roughly) 6000 feet of cable. The same circuit arrangement would
apply to the mid-span extender case wherein the 88 mH coils would
not be present and the gains adjusted for X feet of cable (X could
be in the neighborhood of 10,000 feet).
The natural locations where a repeater or extender can be deployed
are those where the telephone company already has manholes or other
construction where equipment can be placed and where maintenance
personnel have access. The manner in which loop plant has
historically been engineered called for such natural locations at
points 3 kft, 9 kft, 15 kft., etc. (6 kft spacing) from the central
office. Intermediate locations may be accessible in some cases such
as where the telephone cable is carried in an aerial manner, using
telephone poles, rather than in a buried or underground fashion.
Furthermore, the separation rule of 6 kft may not be followed
exactly based on availability of right-of-way and land for the
telephone company to construct manholes or environmentally
controlled vaults.
The fundamental premise of the extender is to put amplification
in-line with the signal. In lay terms, the extender compensates for
the loss of X kft of cable. In a manner of speaking, the
combination of X kft of cable and the extender is equivalent to 0
kft of cable. However, this is not entirely true for the following
reasons. First, the compensatory gain applied may not compensate
for different lengths of cable in the upstream and downstream
directions. Second, the filtering action implied renders some
portion of the spectrum unavailable for transmission of
information. Third, the frequency response of the cable is not flat
with frequency and is determined by the parameters of the cable,
which is a distributed-component system, and thus, since the
filters must have a predetermined frequency response characteristic
using lumped components, the roll-off compensation may not be
exact. For purposes of illustration, however, this rudimentary
model will suffice.
The nature of the ADSL transmission units, the ATU-R (remote) at
the subscriber end and the ATU-C (central) at the Central Office
end, is such that they normally function with loops that are less
than about 12 kft in length. That is, for a nominal data rate,
typically 1536 kbps downstream and 128 kbps upstream, the loop
length must be kept to less than 12 kft. There is some difference
of opinion in the industry as to whether the loop length limit for
such data rates is indeed 12 kft or whether it is 18 kft.
Nevertheless, it is well understood that longer the loop the lower
is the supportable bit rate. In practice we can find loops that are
longer than 12 kft that support higher bit rates than mentioned but
it is also true, based on anecdotal evidence, that there are loops
shorter than 12 kft that cannot support these bit rates.
Considerable variation in bit rates can be observed depending on
the nature of the cable plant, the mix of wire gauges, the presence
of bridged-taps, the mix of traffic carried in adjacent cable pairs
in the same bundle, and so on. Every Telephone Company has its own
Operating Procedures for designating distance and supported data
rate; 12 kft with 1536/128 kbps (downstream/upstream data rates)
are mentioned here as being quite typical. Thus if we design the
extender and/or repeater to compensate for 12 kft of cable, then we
can, in theory, provide for loops as long as 24 kft (somewhat less
in practice for the reasons given earlier). However, if we have a
fixed design, with fixed amplifier gains, then the deployment of a
repeater at 12 kft in the case where the subscriber unit is
slightly further than 12 kft may be problematic for reasons that
will be described below. It is this drawback that we discuss next
as well as indicate how our proposed method provides the solution
to the problem.
In order to compensate for 12 kft of 26-gauge subscriber cable,
typical values for gain are 35 dB in the upstream direction and 48
dB in the downstream direction. These values of gain are arrived at
by computing an "average" loss in the cable taking into account the
relevant frequency bands of transmission in the two directions.
Clearly the filters in the repeater that separate the two
directions must provide adequate cross-over loss to prevent the
repeater circuitry from exhibiting an oscillatory behavior mode.
Now the repeater is going to be some distance from the central
office that can be predetermined. The distance between the repeater
and the subscriber is, however, not constrained. Thus, if the
repeater is very close to the subscriber unit, (i.e., the ATU-R),
then the attempt to provide 35 dB of gain may result in a signal
level that is too high for the circuitry in the repeater to handle
without going into a saturation mode. This is a fundamental
problem. That is, if a fixed gain is specified for the upstream
direction, then the dynamic range required in the upstream
amplifiers, in order that a close-in subscriber unit does not cause
saturation, may be excessive.
Some typical signal levels are provided here to illustrate this
point. The maximum output power of the ATU-C is specified in terms
of a power-spectrum mask which sets the upper limit as roughly
-36.5 dBm/Hz, assuming a 100 ohm termination, over a frequency
range between 30 kHz and 1.1 MHz. This corresponds to a worst-case
(maximum) signal power of about 250 milliwatts or 24 dBm. This
implies that the rms ("root-mean-square") voltage is 5 volts. In
order to support a signal of this strength without going into
saturation, the amplifier needs to have a dynamic range
(specifically a maximum output amplitude) of much larger than 5
volts. This maximum is based on the nature of the signal and is
quantified in terms of the "crest factor". It should be noted that
even though the ADSL standard specifies the maximum power (ATU-C)
via a spectral mask that corresponds to a maximum power of 250
milliwatts, it is common telephone company practice, and therefore
followed by several manufacturers of ATU-C equipment, to limit the
power to about 100 milliwatts.
The crest factor of a signal is defined as the ratio of the
peak-value to the rms-value and is often expressed in decibel
notation. For example, the crest-factor for a sine-wave is 3 dB or,
equivalently, the ratio of peak value (the amplitude of the
sine-wave) and the rms value is 1.414 (actually the square-root of
2). The waveforms generated by ADSL modems tend to be more "peaky"
in nature than sine-waves and it is generally agreed that the
crest-factor applicable for ADSL signals is between 12 dB and 16
dB. The reason for this range is that the notion of "peak" value is
not well defined for random signals and is based on probability.
The notion of "peak" value is that value such that the probability
of the signal voltage exceeding that value is very small. Depending
on what is considered a "small" value of probability, the notion of
peak value, and thus crest-factor, may differ. In the following
discussion we will generally assume a crest-factor of 16 dB and
indicate wherever we deviate from this choice. With this choice, an
ADSL signal of power x dBm will have the same peak value as a
sine-wave of power x+13 dBm. This equivalence facilitates the
computation of required dynamic range since the relationship
between power and amplitude is well defined and commonly understood
for a sine-wave.
For an ADSL signal of 24 dBm, the dynamic range of the amplifier
(output) must be adequate to support a sine-wave of 37 dBm and
considering an impedance of 100 ohms, the amplitude of a 37 dBm
sine-wave is 31.7 volts. Since we require the source impedance to
be 100 ohms as well, the actual dynamic range required for the
amplifier that is driving the 100 ohm load through a 100 ohm source
impedance is .+-.64 volts.
The nominal maximum power of the ATU-R signal, at the ATU-R itself,
is specified as a spectral mask of roughly -34.5 dBm/Hz over the
frequency range of interest (roughly 100 kHz) and this translates
to a power level of about 15.5 dBm (about 35.5 milliwatts). The
peak-equivalent sine-wave power is 28.5 dBm which translates to an
amplitude approximately 12 volts. If the repeater is 0 kft from the
ATU-R and the repeater intends to provide 35 dB of gain in the
upstream direction, the circuitry must be capable of providing peak
voltage levels of the order of 1000 volts! This is not economically
feasible with any technology available today. It should be noted
that even though the ADSL standard specifies the maximum power
(ATU-R) via a spectral mask that corresponds to a maximum power of
35.5 milliwatts, it is common telephone company practice, and
therefore followed by several manufacturers of ATU-R equipment, to
limit the power to about 10 milliwatts.
The solution to this problem is provided here. In particular, we
show that it is feasible to design the repeater such that it is
optimized to provide the longest loop length assuming a mid-span
approach is used. We shall assume that the dynamic range of the
amplifiers is .+-.12 volts for specificity though it will be clear
what alterations need to be made to accommodate any other voltage
range. With this choice, the maximum sine-wave power that can be
delivered through a 100 ohm source impedance into a 100 ohm load is
22.6 dBm. Assuming a crest-factor of 16 dB, the peak-equivalent
maximum ADSL signal power is 9.6 dBm (about 9 milliwatts); assuming
a crest-factor of 13 dB, the peak-equivalent maximum ADSL signal
power is 12.6 dBm (about 18 milliwatts).
The notion of automatic gain control (AGC) is well understood and
is used in a wide variety of applications. AGC is achieved by
monitoring the signal strength of the output signal, usually in
terms of peak value, and thereby generating a control signal
(voltage) for adjusting the gain of a
voltage-controlled-variable-gain amplifier. This notion of
providing AGC is prescribed for the ADSL Repeater. The principle of
the scheme is depicted in FIG. 6.
Referring to FIG. 6, an extender 600 includes an AGC (automatic
gain control) capability. A source 610, either the ATU-R or the
ATU-C, is modeled as a signal generator. The intervening cable
between the source and the extender will have a certain loss
depending on length, wire-gauge, and signal frequency. This loss is
depicted in FIG. 6 as a block 620 labeled "Cable loss". For ADSL,
the standard source and input impedances are both 100 ohms. Hence
we have shown blocks 625, 630 representing 100 ohm source/load
impedances, respectively. A peak detector circuit 640 comprises
circuitry to establish the peak, including smoothing, and in
conjunction with an automatic gain control loop filter 650 (AGC
Loop filter), a signal for establishing the gain of the amplifier
chain. An "AGC Control Voltage" 655 is generated by the peak
detector circuit 640. The AGC control voltage 655 is fed to a
variable gain amplifier 660. The actual amplification is depicted
as a single block though it may be implemented as a cascade of
amplifiers. The key characteristic is that the overall gain is a
function of the control voltage. By adjusting the control voltage
we are adjusting the gain of the amplifier 600 (or chain of
amplifiers). In essence, the gain of the amplifier 660 is
controlled such that the peak excursions of the output signal are,
roughly, equal to a prescribed value. For example, if the amplifier
output voltage 655 range is .+-.12 volts, then the gain is
controlled such that the peaks rarely exceed .+-.11.5 volts and
thus the probability of the amplifier 660 going into saturation is
extremely small. It is common practice with AGC amplifiers to
preset minimum and maximum gain values so that the gain never falls
below the minimum specified value and nor does it ever exceed the
maximum prescribed value.
A similar model is applicable for both the upstream and downstream
directions. That is, we can prescribe AGC for the upstream and
downstream directions independently and thus we will be providing
the "maximum possible" gain in each direction. This is the
"optimal" architecture. The drawback is that separate peak detector
and loop filters are required for the two directions. If power
dissipation and/or material cost constraints are severe, then the
enhancement proposed immediately below is valuable. Specifically,
we note that the control voltage derived by monitoring the output
amplifier of the upstream direction can be used to adjust the gain
of the amplifier gains in both the upstream and downstream
directions.
The amplifier chains in the upstream and downstream directions are
designed to provide a certain maximum gain. This maximum gain is
related to the characteristics of the filters that separate the
upstream and downstream spectral regions so that, as discussed
previously, the repeater does not exhibit an oscillatory behavior.
For example, the nominal maximum gain of the downstream direction
can be set at 56 dB and the nominal maximum gain in the upstream
direction can be set at 41 dB. These values are appropriate to
compensate for, approximately, the loss introduced by 14 kft of
26-gauge subscriber cable. As mentioned before, we assume that the
circuitry is capable of supporting a peak signal of 12 volts
without saturation. That is, the amplifiers can drive a sine-wave
power of about 22.6 dBm (125 milliwatts) into a 100 ohm load
through a 100 ohm source impedance. The corresponding ADSL signal
power (peak-equivalent) is 9.6 dBm (9 milliwatts). If we can assume
that the crest-factor is somewhat less than the assumed 16 dB, and
the ATU-R provides a maximum output power of 10 milliwatts, the
repeater amplifiers will not saturate with a gain of 0 dB. That is,
the "minimum" gain setting in the upstream direction can be set at
0 dB.
To establish the minimum gain in the downstream direction we shall
assume that the ATU-C maximum output power is 100 milliwatts (20
dBm). Further we assume that the loop length between the extender
and the ATU-C is greater than 9 kft of 26-gauge subscriber loop
cable. The nominal frequency range of the downstream signal is
between 160 kHz and 1.1 MHz and over this range the attenuation of
9 kft of cable varies from about 33 dB (160 kHz) to 70 dB (1 MHz).
We believe that for purposes of this power calculation, an
"average" attenuation of 44 dB is appropriate. That is, the maximum
input (ADSL) signal power at the extender, from the central office
side, is roughly -24 dBm. Considering that the output amplifiers
can support an output (ADSL) power level of 9.6 dBm, the "minimum"
gain setting for the downstream amplifier chain can be prescribed
as 33 dB (or 33.6 dB to be more precise). It turns out that 33 dB
of gain, from the viewpoint of extender functionality, is
appropriate to compensate for about 8 kft of 26-gauge subscriber
cable.
The design parameters for the peak-detector and variable-gain
amplifier are such that in the upstream direction the gain of the
amplifier chain is controlled such that, on the average, the peak
signal of the upstream output amplifier is greater than
(12.sub.--.DELTA.) volts only for a very small fraction of the
time. For example, the peak-detector could provide a voltage that
is proportional to the time that the output amplifier output is
greater than 11 volts. This would control the gain of the upstream
amplifier chain as well as the downstream amplifier chain. The
consequence of this feedback operation is that the overall gain of
the upstream amplifier chain will settle at a value such that the
peak value exceeds 11 volts only a small fraction of the time and
thus the probability of the amplifier going into saturation is
extremely small.
We can summarize the design methodology for the Mid-Span Extender
in a qualitative fashion as follows: the approach is well suited
for a Mid-Span Extender for long loops where the specific
positioning of the Extender in the loop is flexible. For example,
the extender may be placed anywhere between 9 kft and 15 kft from
the central office. A method of operating the invention can include
setting the maximum gain in each direction to compensate for some
length of cable (14 kft in the above explanation). The method can
include monitoring the upstream signal at the output of the
(upstream) amplifier chain. Based on the strength of this signal
(basically peak value), the method can include adjusting the gain
in both directions. The AGC philosophy tries to ensure that
clipping does not occur in the upstream direction. Since the
extender is at some minimum distance from the central office,
clipping in the downstream direction is not as important an issue.
The variable gain amplifier in the downstream direction is
configured to have a minimum gain, typically that corresponding to
the compensatory gain for a cable run of slightly less than the
minimum distance expected to the central office.
ADSL Repeater Upstream Automatic Gain Control
Repeaters, as a general class of device for reamplifying and/or
regenerating a signal, often display the characteristic of correct
and compensatory operation over a wide range of incident signal
power levels, such that the variation in transmitted power across
multiple links is much less than the corresponding variation in
received power. The necessity for limited transmitted power
variability is typically due to a combination of regulatory,
technical and fitness-for-use limitations. Regenerating repeaters
which perform an analog-to-digital-to-analog function usually adapt
to receiver power variations in their initial analog stages, and
may use either automatic gain control (AGC) or an adaptive
digitization threshold. Fully-analog reamplifying repeaters are
constrained to use an AGC of some form.
An AGC methodology may be used in a reamplifying repeater for ADSL
signals. Such repeaters, when installed midspan, operate
bidirectionally, amplifying both downstream and upstream
directions, i.e., both toward the customer and toward the central
office, respectively. The deployment of such repeaters can include
constraining their downstream gain within somewhat narrow bounds,
since their distances from the central office are known at the time
of installation. Further, their distances from the central office
may be similar for all repeaters at a particular installation site.
However, the distances between the repeaters and the customer
modems are not correspondingly similar, nor are they necessarily
predictable since repeaters may be installed prior to the
assignment of those repeaters to different customers. The distance
from repeater to modem may range from essentially zero to several
thousand feet, representing a dynamic range of attenuation of over
30 dB within the upstream frequency band. No single upstream gain
characteristic can satisfy both power-limitation and performance
requirements across such a dynamic range. An AGC is the natural
approach to solving this problem. However, there is a confounding
issue, which will be described next.
AGC-ATU Interference in an ADSL Link
The central office DSLAM and the customer modem perform the
functions of ADSL terminal units (ATUs), with the DSLAM being the
ATU-C (central) and the modem being the ATU-R (remote). In
establishing an ADSL link, the ATU-C and ATU-R first operate in a
handshake mode, during which they learn the characteristics of the
path between them, including attenuation and noise margin versus
frequency. This handshake mode causes the downstream and upstream
power levels to fluctuate across a large range, although always
within permissible limits. The ATUs then negotiate the allocation
of their downstream and upstream bit rates to various frequency
bins, in an attempt to optimize performance within their learned
constraints. Once all negotiation is complete, they proceed with
"showtime," when the actual user traffic is carried on the link.
During showtime, downstream and upstream power levels are
essentially stable.
During the handshake process, any variation in the link
characteristics as perceived by the ATUs might easily be
interpreted as a degradation in signal integrity, with a measurable
decrease in signal-to-noise ratio. The result will be either
complete failure to link, or at best a reduction in the negotiated
performance level, manifested as a lower bit rate than could be
achieved in the absence of the variation.
Similarly, from the point of view of the repeater's upstream AGC,
any variation in the upstream signal level, such as occurs during
handshake, may cause the AGC to attempt to counteract the variation
by changing its gain.
To summarize these points, the AGC can interfere with the handshake
of the ATUs, and vice-versa. There is a definite danger that this
mutual interference will either completely prevent establishment of
a link, or result in much worse performance than can theoretically
be achieved. Reduction or elimination of this risk requires that
the AGC be designed with sufficient sophistication to allow the
handshake to occur without interference, while maintaining its
capability to end up in the optimum state. Finally, good design
requires that the AGC permit the link to be established within a
reasonable time; solutions that avoid interference effects by
merely slowing down the dynamics of the AGC are relatively simple,
but may take inordinately long to settle, depending on the number
of gain steps provided, and the particular gain selection algorithm
used. The embodiments of the invention described below satisfy
these requirements.
In the particular embodiment described below, the downstream path
is assumed to have no AGC at all, although the invention can be
implemented in the presence of a downstream AGC. The repeater's
upstream path contains an AGC, which may contain elements of
feedback, feedforward, or both; as such it must contain a
power-measuring capability and a gain-varying capability, with the
latter preferably being controlled by the former. The
power-measuring capability should either directly or indirectly
compare the repeater's upstream output power to at least two
different thresholds. The AGC can cause the upstream output power
during the showtime to have a value between two thresholds, an
upper threshold chosen to guarantee non-violation of the PSD mask,
and a lower threshold chosen to ensure satisfactory performance.
The power-measuring capability can be derived from a circuit (e.g.,
voltage and current sensors) and/or software. The gain-varying
capability should adjust the gain to meet these criteria. Further,
the gain varying capability should produce a gain step smaller than
that represented by the upper and lower upstream output power
thresholds, thereby guaranteeing sufficient hysteresis to avoid
oscillation of the AGC. The gain-varying capability may be either
continuous or discrete, but in any case should not introduce
excessive distortion. The gain-varying capability can be derived
from a circuit (e.g., operational amplifier) and/or software.
In addition to the information from the power-measuring capability,
the invention can also include information about the downstream
power level, provided by a detector. This information is contained
in one bit, which indicates either activity (such as during
showtime and parts of handshake), or inactivity (such as when the
link is down, or during certain other parts of handshake).
The information from the upstream power-measuring capability,
representing as it does at least three distinct states (i.e., above
the upper threshold, below the lower threshold, or between the two)
should include at least two bits. Including the one bit from the
downstream detector, at least three bits of information are thus
available for analysis at any time. To summarize, these three bits
tell whether the upstream power is above the upper threshold, below
the lower threshold, or between the two, and also whether
downstream power is present or not. If more bits are provided, then
this knowledge may be more precise. These bits as a whole can be
termed the "repeater state." These bits as a whole can also be
referred to as the "extender state."
The AGC can also include a link-termination capability, which is
capable of shutting off all power through the repeater in the
upstream direction. The link-termination capability can be derived
from a circuit (e.g., relay) and/or software.
FIG. 7 depicts a simple block diagram of a system that embodies the
invention. An extender 700 includes a CO side connector 710 and a
CPE side connector 720. A downstream amplifier 730 is coupled to
the CO side connector 710 and the CPE side connector 720. A
detector 750 is coupled to an output of the downstream amplifier
730. A upstream amplifier 740 is coupled to the CO side connector
710 and the CPE side connector 720. An AGC control unit 760 is
coupled to the CO side connector 710, to the CPE side connector 720
and between the detector 750 and the upstream amplifier 740. The
AGC control unit 760 can include an AGC algorithm.
The AGC algorithm can dictate how the gain-varying capability
operates according to the repeater state at any time. An objective
of the algorithm is to, if possible, change the gain until the
upstream output power is between the upper and lower thresholds,
then force link termination and reacquisition if necessary to
ensure that the last handshake occurs entirely while the gain is
stable.
In the present invention, the algorithm can include a set of rules
which may be stated generally as follows. If the upstream output
power is persistently below the lower threshold while downstream
power is present, and if the gain can go up, then increase the
gain. If the upstream output power is persistently above the upper
threshold while downstream power is present, and if the gain can go
down, then decrease the gain. If either of the above two rules
would change the gain but for the fact that the gain is at one of
its limits, and if downstream power is present, and if this state
persists, then either witness link termination and reacquisition
since the last gain change, or force it; then keep the gain
unchanged for a while. If the upstream output power is persistently
between the upper and lower thresholds, then once downstream power
is present, either witness link termination and reacquisition since
the last gain change, or force it; then keep the gain unchanged for
a while. If the repeater state persistently indicates a "dead"
line, in that downstream power is absent and upstream output power
is below the lower threshold, then progressively increase or
decrease the gain, reversing the direction of change when the gain
reaches its limits. In all other conditions, leave the gain
unchanged.
A more specific version of these rules is as follows. If downstream
power is present, and upstream power level is below the lower
threshold, and the upstream gain is not at its maximum, and this
state persists for at least a duration (Tup), then cause the
upstream gain to increase. If downstream power is present, and
upstream power level is above the upper threshold, and the upstream
gain is not at its minimum value, and this state persists for at
least a duration (Tdown), then cause the upstream gain to decrease.
If downstream power is present, and the upstream power level is
between the lower and upper thresholds, or if downstream power is
present, the upstream power level is below the lower threshold, and
the upstream gain is at its upper limit, or if downstream power is
present, the upstream power level is above the upper threshold, and
the upstream gain is at its lower limit, and if this state persists
for at least a duration (Tnormal), and downstream power should then
be present, and if the history of the repeater state since the last
gain change does not indicate both at least one instance of absence
of downstream power and at least one instance of an upstream power
level below the lower threshold (not necessarily at the same
times), then terminate the link via the upstream link-termination
capability, until the downstream power is no longer present, or a
maximum duration of link termination (Tshutdown) has elapsed, and
then cease termination of the link, and then keep the gain
unchanged for a subsequent duration (Tsleep). If the upstream power
level is between the lower and upper thresholds, or if downstream
power is present, the upstream power level is below the lower
threshold, and the upstream gain is at its upper limit, or if
downstream power is present, the upstream power level is above the
upper threshold, and the upstream gain is at its lower limit, and
if this state persists for at least a duration (Tnormal), and the
downstream power should then be present, and if the history of the
repeater state since the last gain change indicates both at least
one instance of absence of downstream power and at least one
instance of an upstream power level below the lower threshold (not
necessarily at the same times), then keep the gain unchanged for a
subsequent duration (Tsleep). If downstream power is absent, and
the upstream power level is below the lower threshold, and if this
state persists with the same gain setting for at least a duration
(Tdead), then change the gain in the same direction as the last
change (decrease gain if this is the first change), unless the gain
has reached either its minimum or maximum value, in which case
reverse the direction of change. If any other condition exists,
leave the gain unchanged.
This algorithm is also depicted in FIGS. 8 9. Of course, this
algorithm is just an example of a way in which the invention could
be embodied and the invention is not limited to this
embodiment.
Referring to FIG. 8, the algorithm begins at a start point 800. At
block 810 a determination is made whether the downstream power is
positive. If the downstream power is positive, a determination is
made at block 830 of whether the upstream power is less than a
lower threshold. If the upstream power is not less than the lower
threshold, a determination is made at block 860 of whether the
upstream power is greater than an upper threshold. If the upstream
power is not greater than the upper threshold, the algorithm enters
a wait or force auxiliary operation at block 850. The block 850
will be described in more detail below with reference to FIG.
9.
Still referring to FIG. 8, if the downstream power is not positive,
a determination is made at block 815 of whether the upstream power
is less than the lower threshold. If the upstream power is not less
than the lower threshold the algorithm proceeds to an end at block
899. The end represented by block 899 can be a complete
termination, a dwell period before the algorithm returns to an
earlier point in the algorithm (e.g., start point 800) or a direct
go to an earlier point in the algorithm.
Still referring to FIG. 8, at block 815, if the upstream power is
less than the lower threshold, a determination is made at block 820
of whether a time interval Tdead has elapsed. If the time interval
Tdead has not elapsed, the algorithm proceeds to the end 899. If
the time interval Tdead has elapsed, the gain is incremented at
block 825 in the same direction it was previously incremented,
unless the gain is at a limit in which case the gain is incremented
away from the limit (at first the gain is incremented down). After
block 825, the algorithm proceeds to the end at block 899.
Still referring to FIG. 8, at block 830, if the upstream power is
less than the lower threshold, a determination is made at block 835
of whether the gain is at a maximum. If the gain is at a maximum,
the algorithm proceeds to the wait or force 850. If the gain is not
at a maximum, a determination is made at block 840 of whether a
time interval Tup has elapsed. If the time interval Tup has not
elapsed, the algorithm proceeds to the end 899. If the time
interval Tup has elapsed, the gain is increased at block 845. After
block 845, the algorithm proceeds to the end at block 899.
Still referring to FIG. 8, at block 860, if the upstream power is
greater than the upper threshold, a determination is made at block
865 of whether the gain is at a minimum. If the gain is at a
minimum, the algorithm proceeds to the wait or force 850. If the
gain is not at a minimum, a determination is made at block 870 of
whether a time interval Tdown has elapsed. If the time interval
Tdown has not elapsed, the algorithm proceeds to the end 899. If
the time interval Tdown has elapsed, the gain is decreased at block
875. After block 875, the algorithm proceeds to the end at block
899.
Referring to FIG. 9, the wait or force operation will now be
described in more detail. The wait or force operation can begin at
block 900 after which a determination is made at a block 910 of
whether a state defined by the extender at the time the algorithm
entered the wait or force operation has existed for a time interval
Tnormal. If the state has not existed for the time interval
Tnormal, the algorithm loops back to block 910 to wait. If the
state has existed for the time interval Tnormal, a determination is
made at block 920 of whether a link termination has occurred since
the last gain change. At block 920, if a link termination has
occurred since the last gain change, the algorithm goes to block
970 which will be discussed below.
Still referring to FIG. 9, at block 920, if a link termination has
not occurred since the last gain change, a link termination is
forced at block 930. At block 940 a determination is made of
whether the downstream power is positive. If the downstream power
is not positive, the link is reestablished at block 960. A
determination is then made at block 970 of whether a time interval
Tsleep has elapsed. If the time interval Tsleep has elapsed, the
algorithm proceeds to the end block 999. If the time interval
Tsleep has not elapsed, the algorithm loops back to block 970 to
wait.
Still referring to FIG. 9, at block 940, if the downstream power is
positive, a determination is made at block 950 of whether a time
interval Tshutdown has elapsed. If the time interval Tshutdown has
elapsed, the algorithm proceeds to block 960. If the time interval
Tshutdown has not elapsed, the algorithm loops back to block
940.
In order for this algorithm to function well, the various
predetermined durations must be chosen wisely, and with
consideration of both the typical duration of the handshake process
and the manner in which ATUs such as ADSL modems recover from link
drops. Suggested values which have been tried with favorable
results appear below.
TABLE-US-00001 Duration Suggested (name) value If too low . . . If
too high . . . Tup 3 sec Risk of incorrectly Unnecessarily long
increasing gain total acquisition during handshake time Tdown 0.5
sec Risk of incorrectly Unnecessarily long decreasing gain due
total acquisition to transient noise time, extended PSD violation
Tnormal 9 sec Risk of invoking Unnecessarily long link-termination
acquisition time capability prior to completion of handshake,
possibly causing prolonged link drop Tshutdown 10 sec Risk of not
Unnecessarily long terminating link acquisition time Tsleep 25 sec
Risk of incorrectly Risk of prolonged changing gain if final
operation with link acquisition does incorrect gain not succeed,
further (unexpected, but to adding delay be avoided if possible)
Tdead 10 sec Risk of incorrectly Unnecessarily long changing gain
acquisition time between link attempts
An ADSL repeater without an AGC in the upstream path cannot
optimize its performance across a large range of distances to the
customer's modem; however, an ADSL repeater with an AGC risks
significant degradation of either link acquisition time or
performance, due to the inevitable coupling between the AGC and the
ATUs at each end of the link during the handshake process. The
invention can provide both reasonably short acquisition (often
under 1 minutes, almost always under 2 minutes) and the full
performance benefit promised by a repeater, thanks to an
intelligent algorithm which can have the following attributes. The
invention can recognize power fluctuations occurring during
handshake, and does not over-react to them. The invention can
rapidly perform gain adjustments when sufficient information is
available to warrant them. The invention can recognize that failure
to link can occur due to either insufficient gain or excessive
gain, and has a mechanism to prevent permanent "sticking" in either
condition (namely, the alternating-direction searching method that
occurs when downstream and upstream power are both absent). The
invention can recognize that the process of gain adjustment can
degrade the negotiated bit rate if it occurs during handshake, and
consequently forces a final handshake while the gain is held
stable, unless it is obvious that the last handshake did occur with
stable gain throughout.
The invention can also utilize data processing methods that
transform signals from the digital subscriber loop to actuate
interconnected discrete hardware elements. For example, to remotely
fine-tune (gain adjustment and/or band-pass adjustment) and/or
reconfigure (downstream/upstream reallocation) repeater(s) after
initial installation using network control signals sent over the
DSL.
The invention can also be included in a kit. The kit can include
some, or all, of the components that compose the invention. The kit
can be an in-the-field retrofit kit to improve existing systems
that are capable of incorporating the invention. The kit can
include software, firmware and/or hardware for carrying out the
invention. The kit can also contain instructions for practicing the
invention. Unless otherwise specified, the components, software,
firmware, hardware and/or instructions of the kit can be the same
as those used in the invention.
The term deploying, as used herein, is defined as designing,
building, shipping, installing and/or operating. The term means, as
used herein, is defined as hardware, firmware and/or software for
achieving a result. The term program or phrase computer program, as
used herein, is defined as a sequence of instructions designed for
execution on a computer system. A program, or computer program, may
include a subroutine, a function, a procedure, an object method, an
object implementation, an executable application, an applet, a
servlet, a source code, an object code, a shared library/dynamic
load library and/or other sequence of instructions designed for
execution on a computer system. The terms including and/or having,
as used herein, are defined as comprising (i.e., open language).
The terms a or an, as used herein, are defined as one or more than
one. The term another, as used herein, is defined as at least a
second or more.
PRACTICAL APPLICATIONS OF THE INVENTION
A practical application of the invention that has value within the
technological arts is local digital subscriber loop service.
Further, the invention is useful in conjunction with digital
subscriber loop networks (such as are used for the purpose of local
area networks or metropolitan area networks or wide area networks),
or the like. There are virtually innumerable uses for the
invention, all of which need not be detailed here.
ADVANTAGES OF THE INVENTION
A digital subscriber loop repeater, representing an embodiment of
the invention can be cost effective and advantageous for at least
the following reasons. The invention permits DSL to be provided on
long loops. The invention permits DSL to be provided on loaded
loops. The "Transmux" scheme is superior to the agreed upon
standard, called "DMT", especially in situations where the
separation of upstream and downstream traffic is achieved using
filters; that is, in the Frequency Division Duplexing (or FDD) mode
of operation. The new scheme is especially appropriate for
providing ADSL over long subscriber loops which require "repeaters"
or "extenders". While conventional DSL installation requires that
all load coils be removed from a loop, the invention can include
the replacement of these load coils with what can be termed an
"ADSL Repeater" or "ADSL Extender". In particular, using ADSL
Repeaters (in place of load coils), one particular form of ADSL
that uses the technique of frequency-division-duplexing can be
provided to customers over very long loops. A variation of the
Repeater is the "Mid-Span Extender" where the unit is not
necessarily placed at a load coil site. In addition, the invention
improves quality and/or reduces costs compared to previous
approaches.
All the disclosed embodiments of the invention disclosed herein can
be made and used without undue experimentation in light of the
disclosure. Although the best mode of carrying out the invention
contemplated by the inventor(s) is disclosed, practice of the
invention is not limited thereto. Accordingly, it will be
appreciated by those skilled in the art that the invention may be
practiced otherwise than as specifically described herein.
Further, the individual components need not be formed in the
disclosed shapes, or combined in the disclosed configurations, but
could be provided in virtually any shapes, and/or combined in
virtually any configuration. Further, the individual components
need not be fabricated from the disclosed materials, but could be
fabricated from virtually any suitable materials.
Further, variation may be made in the steps or in the sequence of
steps composing methods described herein. Further, although the
digital subscriber loop repeaters described herein can be separate
modules, it will be manifest that the repeaters may be integrated
into the system with which they are associated. Furthermore, all
the disclosed elements and features of each disclosed embodiment
can be combined with, or substituted for, the disclosed elements
and features of every other disclosed embodiment except where such
elements or features are mutually exclusive.
It will be manifest that various substitutions, modifications,
additions and/or rearrangements of the features of the invention
may be made without deviating from the spirit and/or scope of the
underlying inventive concept. It is deemed that the spirit and/or
scope of the underlying inventive concept as defined by the
appended claims and their equivalents cover all such substitutions,
modifications, additions and/or rearrangements.
The appended claims are not to be interpreted as including
means-plus-function limitations, unless such a limitation is
explicitly recited in a given claim using the phrase(s) "means for"
and/or "step for." Subgeneric embodiments of the invention are
delineated by the appended independent claims and their
equivalents. Specific embodiments of the invention are
differentiated by the appended dependent claims and their
equivalents.
REFERENCES
1. Walter Y. Chen, DSL. Simulation Techniques and Standards
Development for Digital Subscriber Line Systems, Macmillan
Technical Publishing, Indianapolis, 1998. ISBN: 1-57870-017-5. 2.
Padmanand Warrier and Balaji Kumar, XDSL Architecture, McGraw-Hill,
1999. ISBN: 0-07-135006-3. 3. "G.992.1, Asymmetrical Digital
Subscriber Line (ADSL) Transceivers," Draft ITU Recommendation, COM
15-131. 4. "G.992.2, Splitterless Asymmetrical Digital Subscriber
Line (ADSL) Transceivers," Draft ITU Recommendation COM 15-136. 5.
Kishan Shenoi, Digital Signal Processing in Telecommunications,
Prentice-Hall, Inc., Englewood Cliffs, N.J., 1995. ISBN:
0-13-096751-3. 6. The Electrical Engineering Handbook, CRC Press,
(Richard C. Dorf et al. eds.), 1993. 7. ANSI, T1.413-1988.
* * * * *
References